1. Field of the Invention
This invention relates to a very high speed signal processing, more specifically to an optical parametric diffuser for performing wavelength conversion at very high speeds.
2. Description of the Prior Art
A conventional wavelength conversion device will be described first.
FIG. 12 is a block diagram for describing a conventional example. In the following description, it is assumed that the symbols, .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, . . . not only denote wavelengths but also signals of those wavelengths.
The figure shows a multiplexing section 11 for multiplexing two input signals .lambda..sub.1, .lambda..sub.2 and outputting their multiplexed waves .lambda..sub.1, .lambda..sub.2. A mixer section 12 uses a device such as an optical semiconductor amplifier or a differential frequency generator using a KTP crystal. The mixer section 12 receives inputs of multiplexing signals .lambda..sub.1, .lambda..sub.2, mixes them, and outputs signals of wavelengths .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3, .lambda..sub.4 as shown in FIGS. 3(a) and 3(b). The signals of the wavelengths .lambda..sub.3, .lambda..sub.4 are multiplexed waves generated by nonlinear optical effect at wavelengths distant from the input wavelengths .lambda..sub.1, .lambda..sub.2 by the difference .DELTA..lambda. (.DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline.) between the two wavelengths. A band-pass filter (B. P.F.) 13 as shown in FIGS. 3(a) and 3(b) has a passage band wavelength of .lambda..sub.4.
Function of the conventional wavelength conversion device shown in FIG. 12 will be described in reference to FIGS. 3(a), 3(b), 4(a)-4(d) and 5(a)-5(c).
A probe light of a wavelength .lambda..sub.1 and a pumping light of a wavelength .lambda..sub.2 are inputted to the multiplexing section 11. The pumping light is for increasing the nonlinear optical effect. The probe light is a reference light. The multiplexing section 11 multiplexes the two input signals and outputs a multiplexed wave signal to the mixer section 12.
When the multiplexed wave signal is inputted to the mixer section 12, the mixer section 12, as shown in FIG. 5(a), with nonlinear optical effect, outputs multiplexed wave signals of wavelengths .lambda..sub.3, .lambda..sub.4 distant by .DELTA..lambda. from the input signals .lambda..sub.1, .lambda..sub.2. Here, as shown in FIG. 5(c), while multiplexed waves of wavelengths .lambda..sub.5, .lambda..sub.6 respectively distant by 2.DELTA..lambda. from the wavelengths of the input signals .lambda..sub.1, .lambda..sub.2 are produced, since the gain of the mixer section 12 increases in the vicinity of the wavelengths .lambda..sub.1, .lambda..sub.2, and decreases in the vicinity of the wavelengths .lambda..sub.5, .lambda..sub.6, the multiplexed wave components of the wavelengths .lambda..sub.5, .lambda..sub.6 are buried below the noise level as shown in FIG. 5(c) and cannot be taken out as signals.
The band-pass filter 13 has its passage band wavelength at .lambda..sub.4 which is distant by a differential wavelength .DELTA..lambda. from the pumping light wavelength .lambda..sub.2 on the opposite side of the probe light wavelength .lambda..sub.1.
In this way, the probe light signal of the wavelength .lambda..sub.1 is converted to the output light signal of the wavelength .lambda..sub.4.
Conventional mixers for wavelength conversion include those described below.
First, a mixer using an optical semiconductor amplifier will be described.
Inputting two input signals of wavelengths .lambda..sub.1, .lambda..sub.2, outputs of wavelengths .lambda..sub.3, .lambda..sub.4 are produced which are distant by the differential wavelength .DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline. from the input signals on both sides of the input signals.
Here, while either .lambda..sub.3 or .lambda..sub.4 may be used as the mixer output, since the wavelength is not so distant from that of the input signal, even if it is filtered with a band-pass filter of the passage band .lambda..sub.4 as shown in FIG. 3(b), the signals .lambda..sub.1, .lambda..sub.2 are not easy to cut off.
Furthermore, the gain characteristic peak of the optical semiconductor amplifier is in the vicinity of the two input wavelengths .lambda..sub.1, .lambda..sub.2 and the gain is very low in the vicinity of the output signal wavelength .lambda..sub.4. Therefore, conversion efficiency (output signal to input signal) is very low (10.sup.-4 or less).
Next, a mixer using a KTP crystal will be described.
As shown in FIG. 4(a), when two input signals .lambda..sub.1, .lambda..sub.2 as spatial light beams are inputted to a KTP crystal 14, a wavelength-converted output signal comes out as a spatial light beam either in the form of a sum frequency wave, differential frequency wave, or second harmonic wave.
FIGS. 4(a)-4(d) show relationship between the input wavelength and output wavelength (frequency).
FIG. 4(b) shows the case in which the frequency of the output signal is the sum of the frequencies of the two input signals at wavelengths .lambda..sub.1, .lambda..sub.2.
FIG. 4(c) shows the case in which the frequency of the output signal is the difference between the frequencies of the two input signals at wavelengths .lambda..sub.1, .lambda..sub.2.
FIG. 4(d) shows the case in which the wavelengths of the output signals are respectively half the two input signal wavelengths by cutting off the input waves.
When the KTP crystal is used, the process using the spatial light beam requires a certain size and makes installation less easy. Furthermore, with the differential frequency generation, like the optical semiconductor amplifier, since the wavelengths of input and output signals are not largely different from each other, it is less easy to cut off the input signal with a filter.
Furthermore, nonlinear optical effect is not great and therefore the amount of the output signal is small. Therefore, conversion efficiency (output signal to input signal) is very low (10.sup.-5 or less).
As described above, conventionally, when the probe light of the wavelength .lambda..sub.1 and the pumping light of the wavelength .lambda..sub.2 are inputted to obtain the output light of the wavelength .lambda..sub.4 which is apart from .lambda..sub.2 by .DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline., if the .DELTA..lambda. is small, even if a band-pass filter is used, the input wave cannot be removed effectively and the output signal cannot be obtained efficiently.
When the conventional wavelength conversion is applied to the measurement of the probe light of the wavelength .lambda..sub.1, a pumping light of the wavelength .lambda..sub.2 is used to produce an output signal which is apart by the differential wavelength .DELTA..lambda., and the output signal is measured. A disadvantage in that case is that, when the two input wavelengths .lambda..sub.1, .lambda..sub.2 are close to the wavelength .lambda..sub.4, the input wave component contained in the output cannot be completely cut off and the property of the probe light cannot be known accurately.
Since the conventional mixer using the optical semiconductor amplifier or the KTP differential frequency generator is low in efficiency, levels of the pumping light and the probe light must be raised. Furthermore, since the wavelengths of the input and output waves are close to each other, it is difficult to cut off the input wave with a filter. Another problem is that, in order to obtain a large output signal, a large input signal is required.
On the other hand, it is easy to filter and cut off the input signal with a mixer using KTP sum frequency generation or KTP second harmonic wave generation, in which the output wavelength is separated from the input wavelength band. However, such a mixer must handle spatial light, cannot be made compact in size, is very low in efficiency, and therefore is not suitable for practical use.
Furthermore, with the conventional mixer using the optical semiconductor amplifier, the peak of gain is made to agree with the input wavelengths .lambda..sub.1, .lambda..sub.2 as shown in FIG. 5(b), the input signals are mixed and amplified to obtain an output by nonlinear optical effect at a wavelength which is apart by a differential wavelength .DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline.. With this wavelength characteristic, since the peak of gain is near the input wavelength, the more apart from the input wavelength, the lower the gain, and the nonlinear optical effect is less likely to occur. As a result, the output signal of .lambda..sub.5 occurring at a wavelength apart by n.DELTA..lambda. (n is any integer not less than 2) is buried below the noise level and cannot be utilized.